1. Field of the Invention
This invention relates to the fluidized catalytic cracking (FCC) conversion of heavy hydrocarbons into lighter hydrocarbons with a fluidized stream of catalyst particles and regeneration of the catalyst particles to remove coke which acts to deactivate the catalyst. More specifically, this invention relates to the apparatus for performing the FCC process.
2. Description of the Prior Art
Catalytic cracking is accomplished by contacting hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. A high temperature regeneration within a regeneration zone operation burns coke from the catalyst. Coke-containing catalyst, referred to herein as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones, and combusting coke in the regenerator are well known by those skilled in the art of FCC processes. To this end, the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas, respectively.
One well known configuration of FCC unit that gained wide acceptance during the 1960's is a side by side FCC reactor and regenerator. This design comprises a reactor vessel including an upper reaction zone and a subadjacent stripping zone located to the side of a regenerator vessel that contains a single stage regeneration zone. Regenerated catalyst flows from the regeneration vessel through a regenerator standpipe into a riser where it contacts an FCC charge stock. Expanding gases from the charge stock and fluidizing medium convey the catalyst up an external riser and into the reactor vessel. Cyclone separators in the reactor divide the catalyst from reacted feed vapors which pass into an upper recovery line while the catalyst collects in the bottom of the reactor. A stripping zone, formed as a lower part of the reactor vessel, receives spent catalyst from the reaction zone. Steam rises from the bottom of the stripper, countercurrent to the downward flow catalyst, and removes sorbed hydrocarbons from the catalyst. Spent catalyst continues its downward movement from the stripper vessel through a reactor standpipe and into a dense fluidized catalyst bed contained within the regeneration vessel. Coke on the spent catalyst reacts with oxygen in an air stream that ascends through the regeneration vessel and ultimately becomes spent regeneration gas. Again, cyclone separators at the top of the regenerator return catalyst particles to the dense bed and deliver a relatively catalyst-free regeneration gas to an overhead gas conduit.
Changes in regeneration technique, types of available feedstock, and higher throughput requirements have greatly diminished the utility and viability of these stacked arrangements. Since the introduction of the side by side FCC arrangement, two particularly useful additions to regeneration technique include multiple-stage regeneration and the addition of means to remove heat from the regenerator. The major impetus for adopting these changes is the need to improve conversion of a wide variety of feedstocks.
Optimization of feedstock conversion ordinarily requires essentially complete removal of coke from the catalyst. This essentially complete removal of coke from catalyst is often referred to as complete regeneration. Complete regeneration produces a catalyst having less than 0.1 and preferably less than 0.05 wt. % coke. In order to obtain complete regeneration, oxygen in excess of the stoichiometric amount necessary for the combustion of coke to carbon oxides is charged to the regenerator. When a complete combustion of coke occurs the spent regeneration gas contains 1-10% excess oxygen. Excess oxygen in the regeneration zone will also react with carbon monoxide produced by the combustion of coke thereby yielding a further evolution of heat. When CO combustion occurs in a relatively catalyst-free zone of the regenerator, such as the region above the dense fluidized bed in a single regenerator vessel, the resulting high temperatures may lead to severe equipment damage. Such situations may be avoided if the CO combustion takes place in the presence of catalyst particles which act as a heat sink. Therefore, regenerators are generally designed to avoid the combination of free oxygen and carbon monoxide in regions that are relatively free of catalyst. Despite this, the heat evolved from unintended CO combustion may raise the temperature of the catalyst to the point of causing thermal deactivation of the catalyst or may affect the process by limiting the amount of catalyst that can contact the feedstock. The problems of controlling catalyst and regenerator temperatures are exacerbated by the application of FCC processes to crack heavy feedstocks. With the increased coke producing tendencies of these heavy or residual feeds, a complete regeneration of catalyst becomes more difficult due to the excessive heat evolution associated with coke and CO combustion. A common approach to minimizing CO combustion while yet obtaining fully regenerated catalyst has been to perform the regeneration in stages.
Another aspect of FCC operation that is receiving increased attention is spent catalyst stripping. After the catalyst has contacted the feed and prior to its entering the regenerator, the spent catalyst is contacted with steam to prevent the entrainment of hydrocarbon containing gases with the catalyst as it enters the regenerator and to desorb condensed hydrocarbons from the surface of the catalyst. It is now believed that a significant amount of hydrocarbons remain adsorbed on the catalyst as it enters the regeneration zone. The presence of these sorbed hydrocarbons present a two-fold disadvantage in that it reduces potential product yields as well as introducing additional combustible material into the regenerator and thereby raising the temperature of the regeneration zone during coke combustion.
It has been recognized that raising the temperature of the stripping zone can lead to improved stripping results. A convenient source of heat for the stripping zone is the hot regenerated catalyst from the regeneration zone. When mixed in the stripping zone, the much higher temperature of the regenerated catalyst relative to the spent catalyst raises the temperature of the overall temperature of the stripping zone. The higher temperature volatizes condensed hydrocarbons from the surface of the catalyst thereby excluding combustible hydrocarbons from the regenerator and increasing the product yield.
There are drawbacks to the use of fresh regenerated catalyst in the catalyst stripper. The main drawback is the concern that introduction of fresh catalyst into the spent catalyst, steam and hydrocarbon environment of the stripping zone will damage the catalyst or present clean catalyst surfaces that can cause a further loss in hydrocarbon product. Damage to the regenerated catalyst can result from the high temperature steam exposure in the catalyst stripping zone. The very clean regenerated catalyst that enters the stripping zone is highly active so that it may further crack hydrocarbons in the stripping zone or its relatively high surface area can re-adsorb some of the hydrocarbons present in the stripping zone.